Abstract

Phase-matched second-harmonic generation is obtained in various LiInS2 crystals by use of the tunable picosecond output of the free-electron
laser for infrared experiments (FELIX) as the pump source in the mid-IR range
from 2.75 to 6.0μm. Deviations from the phase-matching curve calculated from Boyd’s
refractive-index data are observed. Furthermore, the optical
damage threshold of the crystals has been measured to be 1.1.J/cm2>6GW/cm2 at the 5‐μm wavelength. LiInS2 holds great promise for parametric interaction in the 1–13‐μm range.

Experimental setup for measuring the SHG
phase-matching angles in the 2.75–6.0‐μm range. Type II eoe phase matching in the xy plane is used. From 3.75 to 6.0μm the direct output of the FEL is used, and in the 2.75–4.0‐μm range the frequency-doubled FEL output (doubled in a 7-mm-long ZnGeP2 crystal; see Ref. 7) is used. LWP, long-wave pass filter; SWP’s, short-wave
pass filters. The SH signal generated in the LiInS2 crystal is focused by a BaF2 lens onto the liquid-nitrogen-cooled mercury cadmium telluride
detector (MCT).

Measured internal phase-matching angles
for the (filled circles) rose and (open circles) yellow LiInS2 crystals. The curve is the calculated phase-matching curve based
on Boyd’s refractive-index data (see Ref. 1). The filled triangles
are measurements performed with a similar rose LiInS2 crystal, using a nanosecond optical parametric oscillator as the
pump source (see Ref. 3).

Measured angular dependence of the normalized
SH energy at a pump wavelength of 5.0μm (filled circles) for the yellow crystal. The curve is the calculated
angular dependence of a 2.0-mm-thick crystal based on Boyd’s refractive-index
data (see Ref. 1).